Vacuum degassing apparatus



' Oct. 3, 1967 G. KARLSSON ETAL 3,345,056

VACUUM 'DEGASSING APPARATUS Filed Nov. 16, 1964 i Q U l/ United States Patent ABSTRACT OF THE DISCLOSURE A low-frequency (0.2- c.p.s.) stirring coil for a vacuum tank for degassing metals is located in a chamber closed otT from the tank where it is subjected to a fluid pressure substantially higher than that in the tank,

thus reducing the danger of glow discharge between the coil turns.

The present invention refers amongst other things to means for degassing melts such as steel, comprising a gas tight chamber which can be closed and evacuated, and within said chamber a ladle or other container. The invention also comprises a method for the use of this means.

For some years steel melts and other melts have been degassed under vacuum and simultaneously under stirring by means of an electromagnetic, low frequency, multiphase stirrer. The frequency is 0.2-10 cycles per second, the number of phases being two or more. This method of degassing has been very advantageous, but a problem connected with it has been a tendency for glowing in the stirring windings which occur at the low pressure (below 50 torr, usually around 0.1 torr) at which the degassing is carried out and in which the stirrer thus operates. Such glowing involves amongst others considerable insulation problems and loss of energy.

It has been attempted to avoid the problem of glowing during high frequency heating under vacuum by enclosing the coil with a non-metallic casing. The reason for choosing a non-metallic casing is that the heating and the high voltages have hitherto not made it possible to use any other material for this purpose. It has however been difficult to find a non-metallic casing which is vacuum tight.

The invention describes amongst other things a means by which said disadvantages are avoided and is characterised in that close to the ladle is placed a low frequency, multiphase electromagnetic stirrer in a space sealed off from the chamber, which space is enclosed by a metallic casing round the stirrer, preferably of non-magnetic stainless steel, which space is intended to be kept at a higher pressure than that of the chamber. With this arrangement the stirrer will be able to operate under a pressure which is high enough to avoid tendencies to glowing. At the same time the stirring energy is not diminished in the melt and effective degassing is produced. The stirrer may operate at a pressure between a certain underpressure (such as 50 torr) and atmospheric pressure. The chamber pressure is 0.l50 torr. At the upper limit the pressure in the stirrer space is kept near or at atmospheric pressure. No appreciable heating of the casing is produced. The casing around the space for the stirrer should preferably be metallic, as such casings are of simpler construction and at the same time can be vacuum tight. They are also not energy consuming, preferably such materials as nonmagnetic steels, suitably stainless steel, for example of 18-8 type. It has been observed that during stirring there are hardly any energy losses with such casings of the type mentioned.

It is not necessary, as with induction coils which are 3,345,056 Patented Oct. 3, l 967 enclosed in similar spaces, to use special material in a metal casing with the object of preventing the casing from forming a short electrical circuit. Because of the low frequencies (0.2-10 cycles per second) which are present in the stirrer it is not necessary to arrange any such slits in the casin g around the stirrer.

In the describedembodiment the stirrer is arranged close to one or several cooling fans, arranged to blow cool air along and/or through the stirrer.

The invention also includes a method for degassing melts such as steel by using the above mentioned means. A ladle or other container containing a melt is placed in the vacuum chamber, which is then closed and evacuated to a pressure of less than 50 torr, suitably about 0.1 torr. The method according to the invention is characterised in that the melt is stirred by means of an electromagnetic stirrer placed in the chamber, the stirrer being enclosed in a space separated from the chamber maintained at a pressure higher than the pressure in the rest of the chamber, suitably between 50 torr and atmospheric pressure.

The invention is exemplified in the accompanying figure, which shows a ladle 11 for molten steel 12, which is placed in a chamber 16, which is closed in a gastight way (at 13) and which is intended to be evacuated to a pressure preferably of about 0.1 torr, and at least lower than 50 torr, by means of one or several evacuation pumps, not shown. The evacuation is carried out through one or several pipe 14 in the direction shown by the arrow. The latile is placed on a movable carriage 15 or another moving support which after opening of the chamber 16 is movable with the ladle to a suitable pouring place. The ladle can of course also be moved in the usual way such as by means of a lift hook and travelling crane. Close to the ladle 11 beside and/or beneath it, are arranged one or several stirrers 17, suitably of air cooled or water cooled type. The stirrer 17 is enclosed in a casing 18 (non-magnetic stainless steel, for example of 18-8 type) and as the pressure produced in the space is connected to the atmosphere (at 19), to the chamber 16 through a valve 28 opening or closing at a certain pressure (such as 50 torr) or to an evacuation pump (not shown) for effecting an under pressure around or about 50 torr, the pressure in the space may be kept at a value at which no glowing tendencies occur, which otherwise would exist if the pressure in the space was 0.1 torr or higher, but less than 50 torr. Beneath or beside the stirrer 17 is placed a cooling fan 20, intended to blow cool air along and/or through the stirrer. The tubes to the stirrer 19 and the cooling fan 21 may be provided with lids 22, 23, for example to maintain a certain under pressure and/or to produce a closed cooling air circulation during the degassing process.

The stirrer 17, which usually consists of a coil 24 and core 15, is provided with longitudinal cooling air channels. The electrical energy to the stirrer is supplied by the cables 26, drawn through vacuum tight terminals 27. Between the space in front of the casing 18 and the chamber 16 the valve 28 is placed as mentioned which is opened and closed respectively at for example a pressure of 50 torr in the chamber.

The device is used in the following way:

The ladle 11 filled with molten steel 12 is placed into the chamber 16 which is tightened and evacuated to a pressure of about 0.1 torr, which takes some minutes. The stirring is started by switching on current to the multiphase, low frequency (0.2-10 cycles per second) stirrer 17, which before the placing in of the ladle is cooled by means of the cooling fan 20 (the cover 23 being removed). By means of the valve 28 which is open up to 50 torr, the space within the casing is evacuated to this pressure simultaneously with the evacuation of the chamber 16 (the covers 22 and 23 being now in position) and the pressure in the space for the stirrer is during degassing kept to this value. If the pressure increases because of leakage in the space for the stirrer, the valve 28 is opened and evacuation of the space to 50 torr is restarted. If desired, the valve 28 can be closed and the cover 22 taken away, so that the stirrer will operate under atmospheric pressure.

The cooling in a closed path can, however, possibly also be carried out during degassing.

When the degassing of the melt is completed (this takes for example about 15 minutes) the chamber 16 (at 13) is opened, the carriage 15 with the melt is rolled to a pouring place and the cooling of the stirrer is started can also be applied to furnaces with heating devices, such as electrodes, or on tippable vacuum furnaces.

This and other variations of the invention embodiments may well be feasible within the scope of the invention, according to the claims below.

I melts comprising 4 We claim: 1. Means for degassing molten metal such as steel a gas-tight closable, evacuable tank, a ladle positioned in said tank, means in said tank forming, close to said ladle, a space gas-tightly sealable in relation to said tank, a low frequency, multiphase, electromagnetic sti-rrer in said space, and exposed to fluid there in said space forming mean-s comprising a metallic shell closed around the stirrer, and means for connecting said space with a fluid pressure higher than the vacuum in the tank.

2. Means according to claim 1, in which said metallic shell is composed of non-magnetic stainless steel.

3. Means according to claim 1, in which the connecting means is connectable to the atmosphere at the same time as there is vacuum in the tank, and valve means between said space and the interior of tank for closing at a pressure above the vacuum pressure of the tank.

4. Means according to claim 1, in which at least one cooling fan for blowing cooling air along the stirrer is operatively associated with the stirrer.

References Cited UNITED STATES PATENTS 4/1965 Taylor 22-73 XR 2/1966 Taylor 2273 XR 

1. MEANS FOR DEGASSING MOLTEN METAL SUCH AS STEEL MELTS COMPRISING A GAS-TIGHT CLOSABLE, EVACUABLE TANK, A LADLE POSITIONED IN SAID TANK, MEANS IN SAID TANK FORMING, CLOSE TO SAID LADLE, A SPACE GAS-TIGHTLY SEALABLE IN RELATION TO SAID TANK, A LOW FREQUENCY, MULTIPHASE, ELECTROMAGNETIC STIRRER IN SAID SPACE, AND EXPOSED TO FLUID THEREIN SAID SPACE FORMING MEANS COMPRISING A METALLIC SHELL CLOSED AROUND THE STIRRER, AND MEANS FOR CONNECTING SAID SPACE WITH A FLUID PRESSURE HIGHER THAN THE VACUUM IN THE TANK. 